There are various designs for electronically commutated motors. One known classification system is based on the number of current pulses delivered to the stator winding of such a motor for each rotor revolution of 360° el. A distinction can therefore be made between one-pulse motors, in which only a single driving current pulse is delivered during one rotor revolution of 360° el.; two-pulse motors, in which two stator current pulses, which are usually spaced apart in time from one another, are delivered during one rotor revolution of 360° el.; and also three-pulse, six-pulse, etc. motors.
Such motors are further classified according to their number of stator winding strands, i.e. as single-stranded, double-stranded, triple-stranded motors, etc.
For complete definition of a design, the number of stator winding strands and the number of pulses per 360° el. must therefore be indicated, e.g. a single-stranded, two-pulse motor. Borrowing from the terminology of motors that are operated with alternating or three-phase current, two-pulse motors are also referred to as single-phase motors; a single-phase motor can therefore have either one or two winding strands.
In order to control the current in its stator winding strand, a single-stranded motor usually has a bridge circuit in the form of a so-called H bridge in whose transverse connection (or “diagonal”) the winding strand (e.g. winding strand 26 in FIG. 1) is arranged. By appropriate control of bridge circuit 22, the current in the winding strand is controlled so as to produce current pulses that flow through said winding strand 26 alternately in one direction and then in the opposite direction. Between each two such current pulses there is a reversal of the current direction, which is referred to in electrical engineering as a “commutation.”
The motor usually has a permanent-magnet rotor, and the current pulses in winding strand 26 generate magnetic fields which drive said rotor. The torque generated electromagnetically in this fashion has gaps, and these are spanned by an auxiliary torque, e.g. a mechanical auxiliary torque or a so-called reluctance torque; cf. for example DE 23 46 380 C2 and corresponding U.S. Pat. No. 3,873,897, Müller, issued Mar. 25, 1975. There are an almost infinite number of ways to generate such an auxiliary torque.
Motors of this kind are usually operated from a DC voltage source, e.g. from a battery, a power supply, or a rectifier that rectifies the voltage of an alternating or three-phase power network and delivers it to a DC link circuit from which the motor is supplied with DC voltage. A capacitor, referred to as a link circuit capacitor, is usually connected to this link circuit.
When current flows through a winding strand, energy is stored in it in the form of a magnetic field. If the inductance in such a strand is designated L, and the current I, this energy can be calculated using the formulaW=0.5*L*12  (1).
If the current direction in a winding strand is to be reversed (i.e. “commutated”) in order to generate a circulating magnetic field, this stored energy must first be reduced.
When energy delivery to a current-carrying winding strand is switched off, the effect of so-called self-induction at that winding strand is to cause a voltage rise that is brought about by the stored magnetic energy. Very high voltages can be caused as a result. Semiconductor switches having high dielectric strength must therefore be used.
A certain improvement can be achieved by using a link circuit capacitor, which serves to receive, in the form of electrical energy, the magnetic energy stored in the winding strand, and thereby to limit the voltage that occurs at the motor's DC link circuit. This capacitor therefore receives energy during operation and then immediately discharges it again; in other words, a current, also referred to as a “ripple current,” continuously flows in the supply leads of this capacitor during operation.
In terms of material costs, capacitors of this kind represent an economical solution to the aforementioned problem, but relatively large capacitors—usually so-called electrolyte capacitors—are required; their service life is limited, and is additionally reduced by the considerable heating that is caused by the ripple current. This decrease in the service life of the capacitor consequently limits the motor's service life, which could be substantially longer as far as the motor's mechanical elements are concerned. In addition, in smaller motors there is usually not sufficient space for an electrolytic capacitor, and such capacitors must be expensively soldered in by hand, while the other components can be soldered automatically.
A further possibility for limiting the voltage spikes that occur when a winding strand is switched off is to use a Zener diode or, when a FET (Field Effect Transistor) power stage is utilized, to exploit the so-called avalanche energy. Here the energy that is stored upon shutoff in the winding strand that is to be switched off is converted into heat in the aforesaid semiconductor elements. From the viewpoint of the semiconductor elements that are used, this is dissipated power, and components of appropriate performance must therefore be used.
The energy converted into heat is also “lost” and can no longer be used to drive the rotor, i.e. the efficiency of such a motor is lower.